Natural gas, which is composed primarily of methane, is a low-cost C-containing feedstock and one of the most abundant fuels in U.S. Large quantities of natural gas are flared in refineries, chemical plants, oil wells and landfills releasing greenhouse gases CO2 and unburned CH4. It is important to find an efficient and clean processes to use the natural gas reserves. Direct conversion of methane to useful chemicals or fuels is difficult and requires multi-step processes at high temperatures. The economically available route to producing valuable chemicals from methane is via synthesis gas followed by different chemical routes to manufacture the desired chemicals. In a large scale industrial plant, the production of syngas accounts for a large part of the total costs. Therefore, it is very important to develop more efficient and cost effective methods for the conversion of methane to syngas. The commercial process for natural gas conversion to synthesis gas is the steam methane reforming (SMR) process which is an endothermic reaction as shown in reaction [1].CH4+H2O→CO+3H2Hr=+206 kJ/mol  [1]
SMR reaction is conducted in large tubular reactors to achieve high temperatures needed to obtain high yields which contributes to very high energy consumption. After the SMR reaction, water gas shift (reaction [2]) reaction is performed to remove CO and increase the H2/CO ratio:CO+H2O→CO2+H2 Hr=−41 kJ/mol  [2]
Partial oxidation of methane (POM) is a one step process to form H2 and CO at a ratio of 2 (Eq. 3) from methane:CH4+O2→CO+2H2 Hr=−36 kJ/mol  [3]
Unlike steam reforming, partial oxidation reaction may be conducted auto thermally because of the mild exothermicity of the reaction. Another big advantage is that the H2/CO ratio of 2 is ideal for most downstream processes, making partial oxidation of methane a simple, one-step process.
In the catalytic partial oxidation of methane (CPOM) reaction, catalysts such as noble metal (Pt, Rh, Ir, Pd) and non-noble metal (Ni, CO) have been used to convert methane with oxygen (or air) to syngas in a single step process (See Saleh A. Al-Sayari, Recent Developments in the Partial Oxidation of Methane to Syngas, The Open Catalysis Journal, 2013, 6, 17-28). High reaction rates due to very high auto thermal temperatures exceeding 1000° C. have been obtained with the CPOM Reaction 3. The contact times necessary for CPOM is several orders of magnitude shorter than steam reforming. All these advantages make CPOM a promising and better technology than steam reforming to convert natural gas to syngas. Noble metals such as Pd, Ir, Ru and Pt have been reported as catalysts in the CPOM reaction but they are very expensive. Ni-based catalysts, which are less expensive, have been used but the reactivity is less than that with noble metals. In addition, Ni suffers from deactivation during on-stream due to sintering, carbon deposition, solid state reactions and volatilization as metal carbonyls. Another disadvantage with Ni is that it is a suspected carcinogen and additional costs are needed for safe handling of the materials.
In the CPOM process shown in reaction [3], oxygen is provided by air and that requires an air separation unit to provide pure oxygen making the process very expensive. If air is used in the process the synthesis gas will be diluted by N2. In addition, there are safety issues in mixing air with methane in the CPOM process.
To overcome these difficulties associated with CPOM, chemical looping (CL) partial oxidation of methane (see M. Ryden, A. Lyngfelt, t. Matteson, Chemical looping combustion and reforming in a circulating fluid bed using Ni-based oxygen carriers, Energy and Fuels 2008, 22, 2585-97) has been considered. In CL partial oxidation of methane, an oxygen from an oxygen carrier such as metal oxide is used for partial oxidation and reduced oxygen carrier is oxidized with air in a separate reactor so there is no mixing of fuel with air. The major barrier for the CL partial oxidation of methane is the development of a suitable oxygen carrier that performs the selective oxidation of methane to produce syngas without combusting methane. One of the more important criteria is that syngas produced should not further react with the oxygen carrier. It is very critical to select a suitable oxygen carrier for this process.
Various oxygen carriers have been tested in the past but reactivities have been reported to be low. Ni, Fe, La, ceria, perovskites (See U.S. Pat. No. 6,143,203 to Zeng et al, and US Patent Application No. 2008/0164443 to White et al) based materials have shown the best performance. Ni based materials have environmentally issues while Ceria and La based materials are expensive.
In one or more embodiments of the current invention, the use of Group II ferrites such as Ba, Ca, Mg and Sr ferrites as oxygen carriers for chemical looping methane partial oxidation process are described which showed very promising performance. The disclosed calcium, magnesium and barium ferrites do not have any environmental safety issues and can be easily prepared using readily available materials contributing to a low cost.
In addition, in one or more embodiments a process to supply an oxygen stream by metal oxide (e.g. CuO) decomposition (chemical looping oxygen un-coupling or CLOU) is also described. In this process, metal ferrites perform as catalysts for the CPOM process with a continuous oxygen stream from CLOU to continuously convert methane to syngas. The advantage of this process is that it does not require an air separation unit to separate air.
These and other objects, aspects, and advantages of the present invention will become better understood with reference to the accompanying description and claims.